1. Field of the Invention
This invention relates to a color image pickup apparatus having an optical low-pass filter, and particularly to a color image pickup apparatus having an optical low-pass filter suitable for obtaining images discretely by the use of a solid state image pickup element in a video camera, an electronic still camera or the like.
2. Related Background Art
Generally, in a video camera or the like using a solid state image pickup element having a discrete picture element structure, image information is optically space-sampled to thereby obtain an output image.
If in this case, high spatial frequency components higher than a sampling frequency are included in an object, there will be produced many false color signals of structure or color which the object does not have. That is, frequency components which cannot be picked up by a photographing apparatus (frequency components exceeding the Nyquist frequency) cannot be reproduced as image information and can cause so-called waveform distortion, moire fringes, false colors, etc. to be formed.
For this reason, heretofore, an optical low-pass filter has been disposed in a portion of a photo-taking system to limit the high spatial frequency components of an object. The optical low-pass filter used is often one utilizing the double refraction of a rock crystal plate or the like.
FIG. 4 of the accompanying drawings is an illustration of an optical system in which a conventional optical low-pass filter using a rock crystal plate and utilizing double refraction is provided in a portion of a photo-taking system.
In FIG. 4, the reference numeral 44 designates an image pickup lens, and the reference numeral 48 denotes an image pickup device having a light receiving portion 42 comprising a CCD or the like with a color filter array and cover glass 47. The reference numeral 45 designates an infrared light cut filter, and the reference numeral 46 denotes an optical low-pass filter comprising a rock crystal plate and formed integrally with the infrared light cut filter 45. The optical low-pass filter 46 separates an incident light beam into two ordinary and extraordinary light beams to thereby provide a low-pass effect.
Generally, the rock crystal plate has suffered from the problems that it is expensive and that to separate the incident light beam by a predetermined quantity, the rock crystal plate must be made thick to a certain degree.
So, an optical low-pass filter using a diffraction grating which is inexpensive and abounds in mass production, in place of the double refraction plate, has been proposed.
This optical low-pass filter, as is generally shown by 51 in FIG. 5 of the accompanying drawings, uses a lens system 44 interposed between the optical low-pass filter 51 and the image pickup device 48 in or forward of a photo-taking lens system and therefore, the entire device has tended to become complicated in mechanism and optical construction. Further, since this optical low-pass filter may be integrally disposed in the lens, it has been very difficult to systematize the image pickup apparatus so that various interchangeable lenses can be used.
Therefore, it is a conceivable method to integrally form an optical low-pass filter rearwardly of a photo-taking lens, particularly on the cover glass of an image pickup device, as shown in Japanese Laid-Open Patent Application No. 307423/63.
However, if an optical low-pass filter is disposed between a photo-taking lens and an image pick-up device, there has been the problem that although the low-band filter characteristic is satisfied, the depth of field increases to cause the image of a diffraction grating to be created on the picture plane when the aperture of the photo-taking lens is stopped down.
That state is shown in FIG. 6 of the accompanying drawings. Let it be assumed that a light beam which has left a projection pupil 61 is stopped down. Assuming that this light beam is transmitted through an optical low-pass filter 63 and enters the image plane 62, the picked-up image of the optical low-pass filter is projected onto the image plane without being blurred on the image plane. When 10 at this time, the pitch of the diffraction grating is d and the projection pupil distance is L and the distance between the filter and the image plane is l, the pitch D of the projected image of the optical low-pass filter is ##EQU1## The optical low-pass filter has its transmittance varied periodically and therefore, this projected image becomes a light and dark striped pattern having periodic structure of pitch D in the same direction as the optical low-pass filter, and appears in the photographed image.
To eliminate this pattern, it is conceivable that a burden is applied to a signal processing circuit or there is a great limitation in the structure of the diffraction grating or the mounting thereof.
As related art, there is U.S. application Ser. No. 539,175.